Layer-by-Layer Assembly Technique Used to Study Fluorescence Quenching by Gold Nanoparticles
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Foldamer Made Photoswitchable Foldamers are molecules that reversibly switch between folded and unfolded conformations in response to changes in temperature or solvent composition. The dynamic nature of this reversible conformational change makes foldamers suitable for responsive materials in biological and nanoscience applications. Foldamers that respond to light are desirable because the stimulus is noninvasive and its location, intensity, and timing can be controlled. Toward this end, researchers from Max-Planck Institut für Kohlenforschung in Germany have developed the first example of a photoswitchable foldamer (see Figure 1). As reported in the March 13 issue of Angewandte Chemie International Edition (DOI: 10.1002/anie.200503849; p. 1878), MPI researchers A. Kahn, C. Kaiser, and S. Hecht incorporated a photoisomerizable core connecting two strands of a foldamer— amphiphilic oligo(metaphenylene ethynylene)— previously developed and reported in the literature. The trans-azobenzene core mimics a dimer repeat unit of the foldamers. Irradiation at 365 nm converts the azobenzene core conformation from trans to cis. The researchers deliberately chose foldamer chain lengths that are not long enough to individually fold into Figure 1. Irradiation of the photohelices or with the core in isomerizable core (red) in the a cis conformation but center of a helically folded backthat will form a helix bone (blue) induces a reversible when connected, that is, helix-coil transition. (Courtesy of by the core in a trans conS. Hecht and R.S. Stoll.) formation (see Scheme I). The researchers also introduced enantiomerically pure (S)-α-methyl(ethylene glycol) side chains to bias the sense of the helix and thereby allow them to monitor the conformational transition with circular dichroism spectroscopy. While the photochemical helix-coil unfolding transition occurs in seconds, thermal reverScheme I. Molecular structure of sion at room temperature the photoswitchable foldamer takes place over several includes the azobenzene core (red) hours. and the two foldamer strands, amphiphilic oligo(meta-phenylene The researchers said ethynylene) (blue). that the light-triggered foldamer system they developed not only “can provide fundamental insight into the folding and unfolding mechanisms by enabling time-resolved measurements but promises applications in smart delivery devices based on photoresponsive dynamic receptors.” STEVEN TROHALAKI
Layer-by-Layer Assembly Technique Used to Study Fluorescence Quenching by Gold Nanoparticles Functionalized metal nanoparticles are of great interest for fluorescence-based sensing and biomedical diagnosis and treatFor more information, see http://www.mrs.org/bulletin_ads
MRS BULLETIN • VOLUME 31 • APRIL 2006
RESEARCH/RESEARCHERS
ment. In the case of metal–polymer core–shell structures, the fluorescence of the organic dye may be quenched because of complex interactions with the metal core. To study the effect of separation distance on fluorescence quenching, G. Schneider and G. Decher of Institut Charle
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